As a performance indicator of a steel sheet pile wall constructed by fitting together joints of steel sheet piles, there is a geometrical moment of inertia (I) which shows rigidity of the wall. In general, when the geometrical moment of inertia (I) becomes large, an amount of deformation of the wall body when some load, such as earth pressure or water pressure, is applied thereon becomes small.
The geometrical moment of inertia (I) can be made larger by increasing the steel sheet thickness (t) and height (H) of the steel sheet pile, but the cross-sectional area (A) is desired to be made smaller so as to decrease the steel weight (W) from the economical point of view.
On the other hand, when the size of steel sheet pile is enlarged, penetration resistance (R) of the steel sheet pile is made to be increased. The penetration resistance (R) is a major indicator which affects the workability (penetration performance) of the steel sheet pile, and is desired to be made small. Namely, when the penetration resistance (R) is small, the penetration speed of the steel sheet pile and the penetration performance are made to be improved.
The penetration resistance (R) of the steel sheet pile is mainly made of supporting force due to ground resistance and joint resistance. Among them, the supporting force due to ground resistance (distal end+circumferential surface friction) can be artificially lowered to some extent by temporarily lowering the strength of the ground by using a supplementary construction method, such as a water jet construction method.
On the other hand, the joint resistance is caused by frictional resistance between joints themselves, or a joint and soil in the joint.
Usually, because a gap of a few millimeters or below is set between joints, as long as a steel sheet pile is installed under a state of being completely parallel with the previously installed steel sheet pile, theoretically, there must be substantially no friction between the joints themselves.
However, actually the steel sheet pile is not a rigid body so that its cross section gradually deforms by the supporting force due to the ground resistance so as to generate deflection. As a result, the joints are brought into contact with each other so as to generate friction.
Note that, there is a method of applying lubricant on the joint for decreasing the frictional resistance, but its effect is limited because the lubricant is exfoliated by the friction with the joint or soil.
When the joint resistance is generated, there is formed a vicious circle that the steel sheet pile is inclined so as to further increase the friction.
Once such vicious circle has been generated, it is difficult to correct the same. Accordingly, for example, a guide frame is used for preventing the steel sheet pile to be installed from being inclined, and if an inclination or an misalignment is caused, the steel sheet pile is pulled out and installed again.
With respect to such inclination or misalignment of the steel sheet pile, there is a way to suppress the joint resistance by strictly setting the standard of working management, but this simultaneously causes lowering of the working performance.
Moreover, the cause of increasing the frictional resistance due to the deformation of the cross section of the steel sheet pile has not been removed yet, and even when the pulled out steel sheet pile is installed again, this problem cannot be addressed.
As mentioned above, regarding the setting of the cross-sectional shape of steel sheet pile, the economic efficiency and the workability should be taken into account, and in this regard, some methods of setting the cross-sectional shape of the hat-type steel sheet pile are described in, for example, the following Patent Literatures 1 to 5.
Patent Literatures 1 and 2 describe methods of setting a shape for obtaining a cross-sectional performance superior to those of conventional U-type steel sheet pile or a broad steel sheet pile by satisfying both newly defined relational expressions, one is about the flange width (Bf) and the effective width (B), and the other is about the geometrical moment of inertia (I), the height (H), and the effective width (B).
On the other hand, Patent Literature 3 describes a hat-type steel sheet pile whose penetration resistance (R) is minimized by limiting the range of the web angle θ based on a relational expression of the geometrical moment of inertia (I). Similarly, Patent Literature 5 describes a hat-type steel sheet pile which ensures the penetration performance whose setting is made so as to satisfy a relational expression of the geometrical moment of inertia (I), the effective width (B), and the unit weight (W).
Likewise, Patent Literature 4 describes a hat-type steel sheet pile having an enhanced economic efficiency which can be obtained by satisfying both relational expressions, one is about the geometrical moment of inertia (I) and the unit weight (W) of a hat-type steel sheet pile which is set so as to exceed the linear relation of the unit weight (W) and the geometrical moment of inertia (I) of the conventional U-type steel sheet pile, the other is about the effective width (B) and the flange width (Bf).
These hat-type steel sheet piles are directed to those having the effective width (B) of 700 to 1200 mm, the height (H) of about 200 to 350 mm, and the geometrical moment of inertia (I) of about 10,000 to 20,000 cm4/m.